I finally lost my temper and hissed at him through clenched teeth, "Will you please ACT YOUR FRICKIN' AGE!!!" (Yes, I refrained from saying fucking. I'm not completely hopeless.)
He clammed up for a second, looked at me and said:
"Ok. At what age do I begin to frick?"
I laughed so hard I peed my pants. Where did that smartass kid come from anyway?